Are there any real negligible differences between the two versions???
Do not convert the CSP to a MileagePlus Explorer. Keep the CSP, get the sign-up bonus, enjoy it. 3-6 months later, apply for the MP Explorer card and get another sign-up bonus (hopefully the 50k+5k+10k one instead of the 30k+5k+10k offer).
Also, you confused some of the bonus terms. CSP is a 40k bonus. The Explorer is a 30k bonus + 5k for AU +10k after spending $25k in a year (there's also a 50k + 5k AU + 10k offer you should look into).
The 50k bonus offer you're referring to, that's the one available only via applying in-branch, correct?
You can also see the offer online, which is sometimes triggered by having miles in an existing MP account. There's a thread on FT about it. Google it.